1. Field of the Invention
The present invention relates to a lens-fitted photo film unit of a type in which a stop diameter for a taking lens is changeable.
2. Description of the Related Art
A lens-fitted photo film unit manufactured and sold by the assignee of the present application is incorporated with a taking lens, a shutter mechanism and so forth, and is loaded with a photo film in advance. In virtue of such a lens-fitted photo film unit, a user may enjoy photographing easily. In the lens-fitted photo film unit, a stop aperture is fixed and an f-number of the taking lens is constant so that overexposure and underexposure are likely to occur in accordance with brightness of a subject. In view of this, it is considered to use a stop plate provided with an aperture which is smaller than the fixed stop aperture. In this case, a mechanism for changing the stop is incorporated to set the stop plate in accordance with the brightness of the subject.
As shown in FIG. 8, the lens-fitted photo film unit employs a taking lens 3 constituted of two lenses 1 and 2. FIG. 9 shows color aberration of specific light (light between A and B in the drawing) belonging to the incidence light of the taking lens 3. The above specific light passes through an aperture formed in a fixed stop plate 4, and focuses at a point P on a photo film 5. In FIG. 9, a horizontal line represents a position of the incidence light of the taking lens 3, and a vertical line represents an amount of the color aberration at the point P. From this drawing, it is known that the light passing through the underside of the taking lens 3 (the light passing near the point A) tends to have the greater color aberration in comparison with the light passing through the upper side of the taking lens 3.
As shown in FIG. 10, when a movable stop plate 6 formed with a smaller aperture is disposed at an image side of the taking lens 3, a part of the light focusing at the point P is intercepted by the movable stop plate 6 so that an exposure amount is reduced. However, as shown in FIG. 11, an intercepted ingredient relates to only a portion between B and C (portion of dotted lines) which has the smaller color aberration. Thus, the photo film 5 is exposed by a portion between A and C (portion of solid lines) which has the greater color aberration. Hence, image quality becomes worse due to the color aberration in comparison with the case in that the movable stop plate 6 is not disposed.
In view of the foregoinq, it is a primary object of the present invention to provide a lens-fitted photo film unit in which it is possible to change a stop diameter without deteriorating image quality due to color aberration.
It is a second object of the present invention to provide a lens-fitted photo film unit in which an interval between a movable stop aperture and a fixed stop aperture may be reduced.
In order to achieve the above and other objects, the lens-fitted photo film unit according to the present invention comprises a fixed stop plate, a movable stop plate, and a stop changing mechanism.
The fixed stop plate is disposed at an image side of a taking lens constituted of two lenses, and is formed with a fixed stop aperture positioned at an optical axis of the taking lens. The movable stop plate is provided at an image side of the fixed stop plate, and is formed with a smaller stop aperture being smaller than the fixed stop aperture. The stop changing mechanism moves the movable stop plate between a setting position where the smaller stop aperture is disposed at the optical axis, and an evacuation position where the smaller stop aperture is evacuated from the optical axis. In the present invention, the fixed stop plate and the movable stop plate are arranged such that an interval between them is 1.5 mm or less along the optical axis.
In a preferred embodiment, a subject-side face of the fixed stop plate is formed with a first slope which is declined to an image side and toward an edge of the fixed stop aperture.
Further, an image-side face of the fixed stop plate is formed with a second slope which is declined to the subject side and toward the edge of the fixed stop aperture. The first slope is adapted to be longer than the second slope.